Support tox envlist range syntax

Apparently tox supports numerical range syntax in [generative envlists](https://tox.wiki/en/4.31.0/config.html#generative-environment-list), e.g.

```ini
[tox]
envlist = py3{8-14},pypy
```

It would be nice if check-python-versions understood this.

Start by tracing how check-python-versions reads the envlist in tox.ini, using the tox range example in the issue as the initial case. Done means numerical ranges such as py3{8-14} are understood and produce the expected supported Python versions without breaking existing envlist syntax.

Written by the indexing model from the issue text.